1. Field of the Invention
The present invention relates to a magnetic recording medium, in particular, a magnetic recording tape excellent in surface smoothness and electromagnetic conversion property.
2. Disclosure of the Related Art
Conventionally, magnetic recording media have a magnetic layer on one surface of a non-magnetic support, and have a back coat layer on the other surface of the non-magnetic support in order to improve the running durability thereof and others.
In recent years, the recording density of magnetic recording tape has been desired to be made high in order to cope with an increase in the quantity of recording data. In order to make the recording density of the tape higher, the recording wavelength thereof has been made shorter and the magnetic layer has been made thinner.
In the case that the magnetic layer is made thin, the surface roughness of the support is reflected on the surface of the magnetic layer so that the smoothness of the magnetic layer surface is damaged. Consequently, the electromagnetic conversion property of the magnetic layer deteriorates. For this reason, for example, a non-magnetic layer is formed as an undercoat layer on the surface of the support, and then the magnetic layer is formed on this non-magnetic layer.
As the recording wavelength is made shorter, the magnetic layer surface is required to be made smoother from the viewpoint of spacing loss. For this purpose, the surface of the non-magnetic layer as an undercoat layer is also required to be smoother.
As a non-magnetic powder for forming the non-magnetic layer, acicular iron oxide powder is used. In many cases, sintering is caused in the acicular iron oxide powder. If the acicular iron oxide powder wherein sintering is caused is used, the surface of the non-magnetic layer does not become smooth.
Japanese Laid-Open Patent Publication No. 2004-5931 discloses a powder for a lower layer of a magnetic recording medium, which is made of acicular iron oxide particles having an average long axis length of 20 to 200 nm, which has a specific surface area of 30 to 100 m2/g according to the BET method and a powder pH of 7 or less and which comprises 0.1 to 5.0% by weight of P, in order to obtain a smooth non-magnetic layer surface.
Furthermore, Japanese Laid-Open Patent Publication No. 08-194935 (1996) discloses a magnetic recording medium comprising a magnetic layer which comprises a magnetic powder and a binder and is formed on a non-magnetic support, wherein the concentration of potassium ions eluted into water from any one gram from the magnetic layer is 100 ppm or less. This publication, paragraph [0014] states that the concentration of potassium ions eluted into water from “any one gram from the magnetic layer” means the following concentration when lower layers such as a non-magnetic layer and an undercoat layer are formed: the concentration of eluted potassium ions from any one gram of the combination of the lower layers such as the non-magnetic layer and the undercoat layer, and the magnetic layer.